Changing Equipment profiles changes the IBU's???

[BrehmerhausBrewery]

I am working on building an APA extract recipe and was tinkering with the IBU's.  Then I noticed my equipment profile was not set, and when I changed it the IBU's jumped from 74 to 89.

The only difference in the equipment profile was the amount of boil volume & top up water (added to fermenter), so i'm not sure why it changed at all. The initial setting was 5.7 Boil volume & 0 Top up, and the setting I changed to was 4.14 Boil volume & 1.5 Top up water.

I use the second option due to kettle size, and then add water to the ferementer to bring up to 5 gallons and then mix vigorously.

Does boiling the full volume really make a difference in IBU's?

[Scott Ickes]

The boil volume doesn't make a difference, but the specific gravity of the boiling wort does make a difference.  It is why many extract brewers hold 1/3 to 1/2 of their extract until the last fifteen minutes of the boil.  With a lower gravity wort being boiled, the extraction of the Alpha Acids in the hops is more efficient.  You get more IBU's from the same amount of hops, when boiling a lower gravity wort.

However, I noticed that your IBU's went up, when you reduced your boil volume, which would have raised your wort specific gravity in the boil.  I would have expected your IBU's to go down, not up.

I'm a little puzzled by this.

[BrehmerhausBrewery]

I didn't realize that, thanks for the input. Still not sure, like you said, it went up and not down.

I assume you can trial it on any recipe and see it do the same thing though.

My main concern is I want to make a beer in the 60-65 IBU range, and want to make sure I have the right amount of hops at the right time. This equipment profile is throwing me for a loop though.

[BrehmerhausBrewery]

My bad, my brewhouse efficiency wasn't the same, now it makes sense and shows just as you said, the lower wort gravity has a higher hop utilization (higher IBU for same amount of hops).

Thanks again for your help!

[Scott Ickes]

I'm glad that you were able to figure it out.  Good luck with the APA and let us know how it turns out.